The back-to-school season is the biggest shopping time for Wal-Mart other than Christmas. The unions held news conferences and rallies across the United States on Wednesday, demanding the company boost its wages, expand health benefits and adhere to child-labor laws. Protests have intensified in the past year and threaten to hurt Wal-Mart's sales and profit, one investor said.

"Even a hit of just 1 to 2 percent could make a big difference in comparable store sales and earnings," said Patricia Edwards, a portfolio manager and analyst at Wentworth, Hauser & Violich in Seattle. Her firm manages $5.7 billion in assets, including 69,000 Wal-Mart shares, down from about 1.2 million a year ago.

Wal-Mart's declining public image, also hurt by recent discrimination lawsuits, played a part in the decision to sell shares, said Edwards, who doesn't own the stock personally.

Bentonville, Ark.-based Wal-Mart, which is also the biggest private employer in the U.S., didn't return a call seeking comment.

Shares of Wal-Mart fell 38 cents to $48.84 at the New York Stock Exchange on Wednesday. The stock has fallen about 7 percent in the past year.

Wal-Mart had sales of $285 billion in the year that ended in January, an increase of 11 percent over the previous year. The company said last month that it expects August sales at stores open at least a year to rise 3 to 5 percent.

The teachers' unions may help bolster the food workers union's efforts against Wal-Mart. The food workers union, which represents about 1 million U.S. employees, has failed to make inroads in organizing the company. The union, which recently left the AFL-CIO, now says it will move away from organizing and instead focus on building support against Wal-Mart's practices.

"The only thing Wal-Mart is going to respond to is the pressure that the American people will bear on this company," said Chris Kofinis, a spokesman for the union's Wake-Up Wal-Mart group.